This invention relates to the hot water process for extracting bitumen from tar sand. More particularly, it relates to a system for controlling the density or solids content of the primary separation vessel (PSV) underflow and fluidizing it to render it pumpable and non-plugging.
A large proportion of the world's known hydrocarbon reserves exists in the form of tar sands. One large deposit of this material is found along the banks of the Athabasca River of Alberta. The tar sand exists in the form of water-wet grains of sand, sheathed in a film of bitumen. In treating the tar sand to recover commercially useful products, it is first necessary to separate the bitumen from the water and sand.
The method commonly employed to extract the bitumen from the mined tar sand is known as the hot water process. In the first step of this process, tar sand, hot water and steam are fed into a rotating tumbler and mixed therein. The hot water is supplied at a temperature of about 180.degree. F and in amounts sufficient to supply a slurry containing about 20 - 25% by weight water. The residence time within the tumbler is typically four minutes and the exit temperature of the slurry is about 180.degree. F. While in the tumbler, the tar sand disintegrates and the bitumen particles are liberated from the sand.
The tumbler product is passed through a screen to remove lumps and rocks and is then flooded with additional hot water to further disperse the sand and bitumen particles. A typical flooded, aqueous, aerated slurry will have a composition of 7% bitumen, 43% water and 50% solids, and its temperature will be about 160.degree. - 180.degree. F.
The flooded slurry is then continuously fed into a primary separation vessel. This vessel is conventionally a cylindrical settler having a conical bottom. In the vessel, most of the large sand particles (i.e. plus 200 mesh), fall to the bottom and leave through an outlet as a primary tailings stream. Most of the bitumen particles, rise to the top of the vessel and form primary bitumen froth. This froth overflows the vessel wall into a launder for removal.
A middlings stream, typically comprising about 77% water, 21% solids and 2% bitumen, is continuously withdrawn from the intermediate zone of the primary vessel. The middlings stream is processed in a secondary recovery flotation cell to produce secondary froth and a secondary tailings stream.